This invention relates to an apparatus for the discontinuous mixing of at least two materials, at least one of which is a liquid. More particularly, the invention relates to a mixing apparatus which includes a container and a mixer disposed therein, the mixer having a first rotor which is driveable at high speed through a shaft, and a second rotor which replaces a stator and is driven at a lower speed, both the first rotor and the stator having teeth which cooperate by being positioned on mutually concentric but axially separate circles and which move past one another on circles of different radii to define shearing slots.
An apparatus of the type to which this invention relates and of which it is an improvement is described in U.S. Pat. No. 4,107,792. It is a particular feature of the mixing apparatus described in the aforementioned patent that the radially farther outward circle of teeth on the rotor is disposed outside of the radially outwardly located circle of teeth on a stator. It has been found in the arrangement of the rotor relative to the stator as described in the aforementioned patent that, when solids are dispersed in liquids, the dispersal time is reduced to one fifth of the time required when using previously known types of apparatus. The total amount of energy required for a mixing process is simultaneously reduced by 75 to 80 percent of the energy expenditure which would otherwise be required. By disposing the radially outwardly lying circle of teeth of the rotor externally of the radially outwardly lying circle of teeth of the stator, the individual material particles and drops of fluid acquire a high tangential acceleration due to contact with the radially outwardly lying teeth of the rotor after passing the radially outwardly lying shear slots. The high tangential acceleration leads to the formation of pronounced circular flow patterns which guide all the small particles and/or droplets more often into the rotor-stator system so that all the particles are subjected to very high hydrodynamic shear stresses. Many other advantages and favorable effects are derived from the disposition of stator and rotor as described in the aforementioned patent and these descriptions are incorporated in the present disclosure by reference. One of the advantages of the design according to the U.S. Pat. No. 4,107,792 is that cavitation phenomena develop in the shearing slots between the rotor and the stator. Advantageously also, the teeth of the rotor and the corresponding teeth of the stator have the same axial extent and are mounted parallel to each other. Preferably, the teeth on both rotor and stator are in the form of pins.
The use of mixing apparatus in practice has shown that the dispersal of thixotropic materials in fluids is made more difficult because the above-mentioned well-defined circular fluid flows did not appear.
In order to improve the mixing characteristics for thixotropic materials, it has been proposed in a prospectus entitled "Drais Planetary Kneader Mixers", published by Draiswerke GmbH, West Germany, to mount the radially most outwardly lying row of teeth on the stator and, in addition thereto, to provide supplementary mixing tools driven by planetary gears, each supplementary mixing tool being attached to one planetary gear of the planetary gear train. Furthermore, movable strippers are disposed in the vicinity of the interior wall of the container.
The use of this planetary mixer which employs the combination of a movable scraper or stripper, together with the above-described principal mixer consisting of a rotor and stator, has shown that the mixing of thixotropic materials remains unsatisfactory.